1. Field of Invention:
This invention relates generally to the biological treatment of wastewater and particularly to a method and system for optimizing the operation of a trickling filter.
2. Description of the Prior Art:
The biological treatment of wastewater for the removal of oxygen demanding carbon and nitrogen compounds has been in use for many decades, both in the United States and Europe. Although there are several types of fixed film biological systems available, the trickling filter with a rotary distributor is the most common. The trickling filter is used primarily for reducing levels of BOD (biochemical oxygen demand) and TSS (total suspended solids) as well as the oxidation of ammonia to nitrates. Depending on the predominant ambient temperature at which the trickling filters are operated, some of the filters are covered or domed, while others are not. Examples of biological filters can be found in U.S. Pat. Nos. 2,642,394; 3,275,147; 3,596,767 and 4,486,310.
In order to better understand the current status of biological filters in the art, the following publications are of particular interest: Hawkes, H. A. "The Ecology of Waste Water Treatment" published by the MacMillan Co., New York, N.Y. 1963; Albertson, Orris E., "Slow Down That Trickling Filter!" and "Slow Speed Gains Momentum" WPCF Operations Forum, A WPCF publication for wastewater professionals, January and April, 1989, West Germany, Ein Regelwerk der Abwassertechnischen Vereinigung (ATV) Arbeitblatt A135 Section 3.2.2, Tropfkorperbemessung, April 1983, and Parker, et al., JWPCF publication "Enhancing Reactor Rates in Nitrifying Trickling Filters Through Biofilm Control", May, 1989.
The conventional trickling filter utilizes a film of bio-mass fixed on media to remove and aerobically convert organic matter to carbon dioxide, water and additional bio-mass and to oxidize ammonia (NH.sub.4 N) to nitrates. The fixed media generally consists of rock, plastic or wood. The surface area of the media varies from 12-15 ft.sup.2 /ft.sup.3 for rock and wood to 27-45 ft.sup.2 /ft.sup.3 for plastic. New construction of trickling filters uses predominantly plastic as its media at depths of at least 6 ft. to as high as 42 ft.
Wastewater is distributed over the bio-mass fixed to media through an overhead rotary distributor having generally four nozzled arms or spreaders. This insures a relatively even distribution of wastewater over the fixed bio-mass and thereby produces a relatively constant hydraulic and organic (food) loading throughout the upper portion of the filter's surface area.
Microorganisms and other forms of biological life are the active agents for converting the organic carbon and nitrogen into environmentally acceptable end-products. As a result, a number of operating parameters affect the efficient operation of trickling filters. Some of these include temperature, organic loading, aeration, wastewater characteristics, filter depth and bio-mass thickness, to name but a few.
In current practice, it is not uncommon to produce bio-mass thickness of between 0.10 to 0.30 inch (2.5-8 mm) as reported in the Albertson publication, mentioned above. Such bio-mass thicknesses can reduce the aerobic surface area by a factor of 20% or more. The aerobic bio-mass may be only 5-10% of the total bio-mass on the media. The excess or remaining bio-mass (90-95%) serves no useful purpose in organic removal. Instead, the excess accumulations of bio-mass produces numerous operating problems, such as (1) production of odors; (2) provides a haven for flies and snails; (3) reduces the aerobic surface area; (4) causes excess sloughing of bio-mass which in turn causes an imbalance to downstream processes and loss of aerobic bio-mass; and (5) discolors the filter effluent as well as reduced efficiency of BOD.sub.5 and TSS removal.
Although there are a number of factors which affect the performance of trickling filters, such factors generally affect, either directly or indirectly, the growth of its active agent, the bio-mass. Therefore, one can conclude that it would be highly beneficial to control the bio-mass thickness to minimum levels and thereby provide a bio-mass layer which is predominantly aerobic and eliminate, as much as possible, the underlayer of anaerobic bio-mass. In the past, the bio-mass thickness has been partially and often ineffectively controlled by inconsistent and intermittent dosing or flushing of the media. Since trickling filters are normally driven by the hydraulic jetting action of its nozzled arms, any increase in wastewater flow also increases the speed at which the nozzled arms pass over the bio-mass. With such systems, the dosing or flushing of the bio-mass is frustrated.
In 1983, West Germany reported in a publication identified above that the dosing rate or flushing intensity (SK value) of bio-mass was critical in improving the efficiency of trickling filters. SK may be defined generally as the depth of water discharged to the surface of the filter media per pass of a distributor arm. This SK (spulkraft) value is defined by the following formula: ##EQU1## Wherein "SK" represents the flushing intensity per pass in mm/pass, "q+r" represents the total average hydraulic flow in m.sup.3 /m.sup.2/ hr, "a" represents the number of distributor arms and "n" represents the rotational speed in rev./min.
Currently, SK values of between 2-10 mm/pass of an arm are common. Recently, (Albertson publication) SK values of 50-150 mm/pass were found to be highly beneficial. Still more recently, it has been determined that SK values up to 680/mm/pass have produced better results on strong (high BOD loadings) wastewaters. However, this does not mean that still higher SK values would be more beneficial as, at some point, increasing SK values will impair the filter's performance even if the bio-mass thickness is near optimum.
An increase in SK will reduce the retention time of the feed in the trickling filter. This can reduce the operating efficiency of the trickling filter even if the upper portions of the filter possesses optimal bio-mass content; i.e., aerobic and well flushed. This is of concern for filters operating at high levels of efficiency where high retention time (low SK) is needed to provide the most efficient treatment at maximum capacity. It is also recognized that many trickling filters often do not have good bio-mass development in the bottom half of the bio-towers and the growth is considered "patchy" (Parker, et al.). The reason is that the lower half of the media area fails to receive sufficient food (BOD.sub.5 and NH.sub.4 N) as the biomass on the media area above consumes most of it. This prevents the lower portion of the media in the bio-tower to fully develop due to starvation.
Therefore, based on the prior art, there appears to be no specific optimum SK value for any given trickling filter. This is primarily due to the wide range of operating variables which may exist for any given wastewater. It would also appear that the optimum SK value for biological treatment and full bio-mass development will vary substantially from the optimum SK value for flushing out excess bio-mass.
In the patent application heretofore filed, a system was described wherein the trickling filter would be operated at essentially two sets of SK values. One set was used primarily for flushing excess bio-mass from the media and the other was used primarily for optimal removal of BOD.sub.5. Although this type of system works well with most municipal waste water treatment systems, it has now been found that a still more efficient use of the trickling filter can be realized if the bio-mass located in the lower portions of the trickling filter is more fully developed.
As depicted in FIG. 1, the non-shaded area represents the SK values that were used in the above two step system. The shaded portion represents the effect that the SK values used in accordance with this invention would have on full development of biomass throughout the bottom depth.
For example, and as depicted in FIG. 1, if a flushing cycle of four hours at an SK of 300 mm/pass was used, only about 5% of the daily loading (food nutrients) would be available for development of the lower portions of the bio-mass. In order to accomplish the additional objective of developing the lower portions of the bio-mass, it is necessary to control the SK values as a function of the influent loading. This is depicted in FIG. 1 by the shaded area between the two step SK and the modulated SK value over a twenty-four hour period. As shown, modulated SK values vary exponentially with the loading as is necessary to ensure that food is delivered to the lower media zones. The shaded area is representative of the food transmitted to the lower zones as a result of higher SKs during the lower loading periods.